Tunnel electron emitter cathode

ABSTRACT

A high efficiency tunnel electron emitter in which a unique emitting layer is applied to the insulator layer of a metalinsulator-emitting layer structure. The emitting layer consists of a low work function material such as cesium that is deposited directly on the insulator layer, a thin layer of a conductive metal that is deposited onto the cesium, and an exposed surface of the conductive metal having a layer of cesium oxide applied thereto to lower the work function. The amount of cesium deposited on the insulator layer is critical and is monitored for the proper amount during deposit.

United States Patent Caldwell et al. 1 1 Dec. 19, 1972 [s41 TUNNEL ELECTRON EMITTER 3,246,200 4/1966 Kanter ..317/234 s CATHODE 3,321,659 5/1967 Brody ..317/234 1" 3,440,499 4 1969 F t l. ..317 234 T [72] 3,469,184 9/1969 1.22:2: 121. ..317/234 '1 both of Alexandria, Va. 1

[73] Assignee: The United States of America as Primary Examiner-John uc e represented by the Secrghry f th Assistant Examiner-Andrew .1. James Army Attorney-Harry M. Saragovitz, Edward .1. Kelly, Her- [22] Filed: March 18, 1971 bert Her] and Mllton W. Lee

[21] Appl. No.: 125,825 [57] ABSTRACT A high efficiency tunnel electron emitter in which a [52] US. Cl. ..317/234 R, 317/234 S, 317/234 T, unique emitting layer is applied to the insulator layer 317/235 N ,of a metal-insulator-emitting layer structure. The [51] Int. Cl. ..H01l3/00, H011 5/00 emitting layer consists of a low work function material Field of Search ..3 17/234 T, 238; 313/94; such as cesium that is deposited directly on the insula- 315/94; 324/65 tor layer, a thin layer of a conductive metal that is i deposited onto the cesium, and an exposed surface of References Cited the conductive metal having a layer of cesium oxide applied thereto to lower the work function. The UNITED STATES T N amount of cesium deposited on the insulator layer is 2,735,049 2/1956 DeForest ..317/234 T critical and is monitored for the proper amount during 3,184,636 Sl1965 Dore et al. ....317/234 T deposit. 3,184,659 5/1965 Cohen ....317/234 T 3,214,629 10/1965 Apker ..317/234 T 7 (11111118, 3 Drawing Figures e i I FIG.

FIG.3

INVENTORS BRIAN s. MILLER LARRY v. CALDWELL B W A). La ATTORNEYS M 11% FM TUNNEL ELECTRON EMITTER CATIIODE The invention described herein may be manufactured, used, and licensed by or for The Government for governmental purposes without the payment to us of any royalty thereon.

BACKGROUND AND SUMMARY OF THE INVENTION This invention is in the field of solid state tunnel electron emitters. Various solid state materials and their specific sizes, arrangements and electrical potentials applied thereto have been used in ejecting electrons from solid state tunnel electron emitters in a vacuum tube or into the collector of a solid state device.

Typical emitters have an insulator layer sandwiched between two conductive metals with the exposed surface of the second metal having a thin layer of material applied thereto for reducing the work function. A potential source applied between the two conductive metals causes electrons to flow out the first metal and tunnel through the insulator layer into the second metal. These electrons will either recirculate through the potential source back to the first metal or be ejected out the low work function material at the exposed surface on the second metal. Prior art emitters indicate that only a small fraction of the electrons that tunnel through the insulator are ejected out from the second metal. Some typical materials used in the prior art devices are aluminum as the two conductive metals, aluminum oxide as the insulator layer and cesium oxide as the material that lowers the work function. There are other materials, however, that may be used satisfactorily.

Another embodiment of a prior art tunnel electron emitter includes substituting an emitting layer of the following type for the second metal. This prior art emitting layer has a metal, such as antimony, deposited on the insulator and then cesium deposited over the metal. The present inventive structure is formed by applying the cesium onto the insulator first and then applying the metal. The resulting structure has been demonstrated to eject a substantially higher number of electrons out the low work function material than was the case with prior art tunnel emitters.

The thickness of the insulator layer in the present invention is typically less than one electron mean free path. The desired amount of cesium applied to the insulator layer is monitored by a measuring circuit.

This measuring circuit indicates photoemission from the insulator layer as cesium is deposited. Thus when photoemission peaks the cesium deposition is stopped immediately. A thin layer of conductive metal such as antimony, silver, gold, etc., is deposited over the cesium. A low work function surface treatment is applied to the exposed surface of the conductive metal. This surface treatment may be alternate layers of cesium and oxygen.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 illustrates a prior art tunnel electron emitter;

FIG. 2 is a schematic diagram illustrating the structure of the present invention; and

FIG. 3 illustrates the method for applying cesium to the insulator layer and the measuring circuit used to determine the proper amount.

DESCRIPTION OF THE PREFERRED EMBODIMENT With reference to FIG. 1, a prior art tunnel electron emitter is shown with conventional metal-insulatormetal layers 10, 12 and 16 respectively, and a layer of treatment material 18 which is used to reduce the work function of the surface of metal 16. Material 18 may be cesium oxide. A first potential source 20 is applied across the two metals l0 and 16. The electrons from metal 10 tunnel through insulator layer 12 with sufficient energy to be ejected through metal 16 and out layer 18. These electrons that are emitted out from layer 18 may be accelerated by a grid 24 having a positive potential applied thereto from second potential source 22. The number of electrons ejected out layer 18 is a small fraction of the total number of electrons crossing insulator layer 12. The other electrons that tunnel through insulator layer 12 return through source 20 back to metal 10.

An improved metal-insulator layer-emitting layer tunnel emitter cathode is shown in FIG. 2. The specific structure of this improved tunnel emitter allows a much greater number of electrons to be ejected out from layer 18 and to be acceleratedby grid 24. Grid 24 may have several hundred volts applied thereto by second potential source 22. Metal layer 10 may be a good conductor such as aluminum. Insulator layer 12 may be made of aluminum oxide (Al- 0 about Angstroms in thickness. The specific thickness of the insulator layer depends on the insulator material used and the potential of source 20.

The novel emitting layer consists of layers 13, 15 and 18. Layer 14 includes a layer of cesium l3 deposited on insulator layer 12 and a thin layer of conductive metal 15 deposited over the cesium. Cesium l3 and metal 15 are deposited by the conventional vacuum evaporation technique. Metal 15 may be either silver, gold or antimony all of which'have been used with satisfactorily results. The inventors have demonstrated that greatly improved efficiency results from applying the cesium first and then applying the metal over the cesium.

The amount of cesium deposited to form layer 13 is relatively critical. This amount is measured while cesium is being vaporized on insulator layer 12. The critical amount desired is at a thickness where photoemission is greatest from the cesium layer on electrode 42. The method of depositing and measuring the amount of cesium deposited on insulator layer 12 is explained with reference to FIG. 3 herein below.

Cesium metal from some standard cesium source 40 is vaporized in a vacuum environment (with the enclosure of the vacuum environment not shown) allowing a cesium layer 13 to form similarly on both the insulator layer 12 and electrode 42 of the measuring circuit. The measuring circuit includes electrode 42 electrically connected through a current measuring meter 44 and potential source 46 to a collector-ring 50. The positive terminal of potential source 46 is connected to ring 50 to collect electrons from electrode 42. A light source 48 is positioned to allow light beans 48a to fall on electrode 42. The measuring circuit measures photoemission from electrode 42 which is caused by light beams 48a striking the electrode. The vaporized cesium, represented by dashed lines 40a, 40b, and 400, at source 40 forms a cesium layer 13 on electrode 42 and on insulator 12 which is positioned near 42. Both insulator 12 and electrode 42 receive the same quantity of cesium thereon. During the time that layer 13 is'being deposited meter 44 is used to measure the photoemission from electrode 42. Other means of monitoring cesium thickness during deposition may be by mass spectroscopy, quartz crystal mass monitor, etc. The present measuring means is merely illustrative and does not constitute a part of this invention. The cesium source 40 is shut-off when photoemission from the cesium layer on electrode 42 is peaked.

A thin layer of conductive metal 15 is then evaporated over the cesium in the conventional manner. The thickness of this metal layer 15 is on the order of an electron mean free path. A cesium oxide layer 18 is deposited as a surface treatment material on the exposed surface of metal 15 to provide a low work function for electrons passing therethrough. After the layers are formed a first bias potential source 20 is applied between layers 10 and 14 with the negative terminal of 20 connected to metal 10. Source 20 causes a high electric field across insulating layer 12 on the order of l' -l0 volts per centimeter. This high electric field will cause electrons to tunnel across insulator layer 12 from layers and be injected into layers. 14 and 18 with sufficient energy such that a significant percentage of the electrons escape through layer 18. It has been demonstrated that the number of electrons escaping from surface 18 of the present tunnel electron emitter is much greater than that of prior tunnel electron emitters. A second bias potential source 22 applies a positive voltage to grid 24 for accelerating electrons escaping from surface 18 into a vacuum tube.

There are many uses of such a tunnel electron emitter as disclosed herein. Also, various embodiments may be fabricated according to the specific materials used for conductors, insulators, etc., but inventors novel method and apparatus of applying a low work function material such as cesium directly to the insulator layer and then applying the thin metal layer onto the cesium is critical to the successful operation thereof.

In the case of the cold cathode tunnel electron emitter used in vacuum tube or display devices, layer 10 is a metal rich in free electrons, such as aluminum. Layer 10 could, alternately, be an appropriate semiconductor which after exposure to light or some other radiation such as x-rays, atomic radiation, etc., would produce free electrons in the conduction band and allow the semiconductor to act as an electron source.

We claim: 1. An improved tunnel electron emitter including, a metal emitter layer;

an insulator layer, said insulator layer being in intimate contact with said metal emitter layer;

an electron rich emitting source layer in intimate contact with said insulator layer;

said emitting source layer comprises a measured amount of cesium therein forming a low work function integrated overlapping layer with said insulator layer and a thin layer of conductive metal having a thickness of an electron mean free path in intimate contact with said measured amount of cesium; surface treatment material covering the exposed surface of said thin layer of conductive metal for lowering the work function of said thin layer of conductive metal;

an external grid positioned opposite said surface treatment material;

a first potential source connected between said metal emitter layer and. said electron rich emitting source layer for applying an electric field in the forward direction across said insulator layer for tunneling electrons therethrough;

and a second potential source connecting said electron rich emitting source layer and said external grid for accelerating electrons ejected fromsaid electron rich emitting source layer.

2. An improved tunnel electron emitter as set forth in claim 1 wherein said surface treatment material is alternate layers of cesium and oxygen.

3. An improved tunnel electron emitter as set forth in claim 2 wherein said metal emitter layer is aluminum and said insulator layer is aluminum oxide.

4. An improved tunnel electron emitter as set forth in claim 2 wherein said metal emitter layer is a semiconductor in which radiation exposed thereto produces free electrons in the conduction band that reach said insulator layer.

5. An improved tunnel electron emitter as set forth in claim 1 wherein said layer of conductive metal is antimony.

6. An improved tunnel electron emitter as set forth in claim 1 wherein said layer of conductive metal is silver.

7. An improved tunnel electron emitter as set forth in claim 1 wherein said layer of conductive material is gold. 

2. An improved tunnel electron emitter as set forth in claim 1 wherein said surface treatment material is alternate layers Of cesium and oxygen.
 3. An improved tunnel electron emitter as set forth in claim 2 wherein said metal emitter layer is aluminum and said insulator layer is aluminum oxide.
 4. An improved tunnel electron emitter as set forth in claim 2 wherein said metal emitter layer is a semiconductor in which radiation exposed thereto produces free electrons in the conduction band that reach said insulator layer.
 5. An improved tunnel electron emitter as set forth in claim 1 wherein said layer of conductive metal is antimony.
 6. An improved tunnel electron emitter as set forth in claim 1 wherein said layer of conductive metal is silver.
 7. An improved tunnel electron emitter as set forth in claim 1 wherein said layer of conductive material is gold. 